This project will begin to examine the structural changes and the functional domains of the arenavirus glycoprotein complex. The mechanism of fusion pore formation is necessary for all enveloped viruses. Detailed examination of classical class-I fusion proteins have illuminated the fusion mechanisms of influenza, HIV and paramyxoviruses. As evidenced by the 2007 classification of the arenavirus glycoprotein as a class-I fusion protein, details about the structure, conformational changes, and biochemical characterization of these fusion proteins is limited. The arenavirus family in considered an emerging virus, because although the viruses are endemic in the rodent population, new strains continually are identified to infect humans. Some strains, including Lassa and Junin, cause devastating hemorrhagic fevers with high morbidity and mortality. In addition new evidence suggests that Lymphocytic Choriomeningitis virus, endemic to the US and the rest of the world may be responsible for clinically significant disease. There are no effective therapeutics or vaccines currently licensed to treat the infections, and Lassa fever alone affects half a million people each year and is responsible for 5000 deaths in West Africa. Understanding the mechanism of entry, specifically identifying the structural/functional domains within the glycoprotein complex (GPC) will provide new targets for future therapeutics, as well as provide a basic understanding of an understudied class-I fusion protein. Towards this end, this proposal has two Specific Aims: 1) Determine the receptor binding site in the GP1 subunit; 2) Define the geometry of the Lassa fever glycoprotein complex in situ. Utilizing biochemical analysis, as well as cell-to-cell fusion assays, native-electrophoresis, and state-of-the-art cryo-electron tomography, a model of the conformational changes required for the glycoprotein complex to induce fusion will emerge. Future studies using structure based designs will be able to target specific conformations in the GPC to produce effective therapeutics and vaccines. The project offers ample opportunity for me to utilize innovative electron microscopy techniques, and combine the structural data with advanced biochemical approaches to elucidate the mechanism of arenavirus fusion in their natural state. This combined expertise will provide a solid foundatio for my future independent research career at the intersection of molecular virology, computational biology, biochemistry and structural analysis. Reaching my project goals in a collaborative, interdisciplinary approach will successfully prepare me for future leadership of research teams and diverse expertise that will be required to solve increasingly complex and multifaceted biological challenges of the future.